Examples of Multiplexing

Amultiplexeris essentially a very simple switch consisting of a multiplexing function and a demultiplexing function connecting a single trunk port in the network to many access ports connected to individual traffic sources, as illustrated in Figure 4-17. The parallelo-gram symbol with the small end on the side of the single output (called the trunk side) and the large end on the side with multiple interfaces (called the access side) frequently denotes a multiplexer in block diagrams. The symbol graphically illustrates the many-to-one relationship from the many ports on the access side to the single port on the trunk side, as well as the one-to-many relationshipfrom the trunk side to the access side.

The multiplexing function shares the single output among many inputs. The demultiplexing function has one input from the network, which it distributes to many ac-cess outputs. The multiplexing and demultiplexing functions can be implemented by any of the generic switching functions described in the previous section. Usually, the same method is used for both the multiplexing and demultiplexing functions so that the multiplexing method used on each of the interfaces is symmetrical in each direction. Gen-erally, the overall speed or capacity of each port on the access side is less than that on the trunk side. For example, different levels in the time division multiplexing (TDM) hierar-chy operate at increasingly higher speeds by aggregrating multiple lower-speed TDM signals together. We give more detailed examples for each of the generic methods de-scribed in the preceding sections.

Figure 4-16. Point-to-multipoint switching function definitions

Multiplexers share a physical medium between multiple users at two different sites over a private line, with each pair of users requiring some or all of the bandwidth at any given time. Many simple multiplexers statically assigned a fixed amount of capacity to each user. Other multiplexing methods statistically assign capacity to users according to demand to make more efficient use of the transmission facilities that interface to the net-work. You’ll see these calledstatistical multiplexersin the technical literature. TDM is often used to reduce the effective cost of a private access line or international private line by combining multiple lower-speed users over a single higher-speed facility.

Frequency Division Multiplexing (FDM)

Analog telephone networks made extensive use offrequency division multiplexing (FDM) to aggregate multiple voice channels into larger circuit groups for efficient transport.

FDM multiplexes 12 voice-grade, full-duplex channels into a single 48 kHz bandwidth groupby translating each voiceband signal’s carrier frequency. These groups are then further multiplexed into a mastergroup made up of 24 groups. Multiple mastergroup an-alog voice signals are then transmitted over anan-alog microwave systems. A lower-fre-quency analog microwave spectrum was used to frelower-fre-quency division multiplex a DS1 digital data stream in a technique called Data Under Voice (DUV).

58

Figure 4-17. Switching model of a multiplexer

centered around the wavelengths of 1300 and 1550 nm (10 m) as shown in the plot of loss versus wavelength in Figure 4-18 [Personick 85]. The total bandwidth in these two windows exceeds 30,000 GHz. Assuming 1 bps per Hertz (Hz) would result in a potential bandwidth of over 30trillionbps per fiber!

Recall the basic relationshipfrom college physicsλ =c, whereλis the wavelength in billionths of a meter (i.e., a nanometer or nm), is the frequency in billions of cycles per second (gigahertz or GHz), andcis the speed of light in a vacuum (3×10⁸m/s). Applying this formula, the carrier frequency at the center of the 1300 nm window is 2300 GHz and 1900 GHz in the 1550 nm window. The available spectrum for signal transmission is 18,000 GHz in the 1300 nm window and 12,500 GHz in the 1550 nm window, as shown in the figure. Chapter 23 defines the concepts of optical signals and their frequency spectra.

This means that at a spectral efficiency of 1 bps per Hertz, a single fiber pair carrying these two bands could theoretically carry approximately 30 Terabits (1×10¹²) per second of du-plex traffic. The sharp attenuation peak at 1400 nm is due to residual amounts of water

Figure 4-18. Optical fiber transfer characteristic

(an OH radical) still present in the glass. Continuing improvements in optical fiber manu-facturing will likely make even more optical bandwidth accessible in the future. Com-mercial long-haul fiber optic transmission is now using between two and eight wavelengths per fiber, in what is called wideband WDM, in these two windows. Imple-mentations of dense WDM (DWDM), supporting up to 100 optical carriers on the same fiber, were available at the time of writing.

Time Division Multiplexing (TDM)

Time division multiplexing (TDM)was originally developed in the public telephone net-work in the 1950s to reduce costs in metropolitan area netnet-works. It also eliminated FDM filtering and noise problems when multiplexing many signals onto the same transmis-sion medium. In the early 1980s, TDM networks using smart multiplexers began to ap-pear in some private data networks, forming the primary method to share costly data transmission facilities among users. In the last decade, time division multiplexers have matured to form the basis of many corporate data transport networks. The premier exam-ple of TDM is DS1 and E1 multiexam-plexing; Chapter 6 describes this for the ISDN Primary Rate Interface (PRI).